A disc drive is a data storage device used to store and retrieve computerized data in a fast and efficient manner. A typical disc drive comprises one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. The data are stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs. The heads are supported over the corresponding disc surfaces by air bearings set up by the rotation of the discs. A servo circuit controllably positions the heads through the application of current to an actuator coil which forms part of a voice coil motor (VCM).
To maximize data transfer performance, efforts are made to minimize the time required for to move the heads from one track to another during a seek operation so that the disc drive can proceed to transfer data to or from the various tracks. For seeks above a certain length, a velocity controlled approach is typically used wherein a velocity profile is selected to define a desired velocity trajectory for the head. To carry out the seek, current is applied to the actuator coil to first accelerate the head in the direction of the destination track until a maximum velocity is reached. At a predetermined distance from the destination track, current of opposite polarity is applied to the actuator coil to decelerate the head to reach the destination track. During the deceleration phase of the seek, the velocity of the head is repeatedly determined and compared to the corresponding desired velocity as set forth by the velocity profile; corrections to the current are made in order to cause the head to follow the deceleration trajectory to the destination track. The desired velocity at any given sample point of the velocity profile is referred to as the "demand velocity."
While the aforementioned velocity-controlled seek methodology has been found useful in the disc drive art, problems have arisen in that sudden, large changes in current applied to the actuator coil can establish undesired vibrations in the disc drive, adversely affecting servo performance as well as generating unwanted acoustic noise. As a result, various improvements have been proposed in the prior art to minimize such effects.
For example, U.S Pat. No. 5,475,545 issued to Hampshire et al. and assigned to the assignee of the present invention, discloses a servo circuit which is used to control the position of heads of a disc drive using a modified velocity-controlled approach. As will be appreciated, at the beginning of a velocity-controlled seek, the actual velocity of the head is substantially zero, whereas the velocity profile requires a demand velocity of substantially maximum velocity. This results in a large velocity error, and the servo circuit responds by applying the maximum available current to the coil, resulting in quick acceleration, but also large amounts of vibration. Accordingly, the Hampshire U.S. Pat. No. 5,475,545 reference modifies initial portions of the velocity profile by providing an initial demand velocity of zero and then incrementing the demand velocity during each of a succession of time periods by a fraction value until the maximum demand velocity is reached. In this way, the velocity errors at initial stages of the seek are reduced and the current is more gradually applied to the VCM until the maximum velocity is reached. By selecting an appropriate fraction value, the seek can still be carried out in a short amount of time, but with a substantial reduction in acoustic noise.
U.S. Pat. No. 5,657,179 issued to McKenzie et al. and assigned to the assignee of the present invention, provides an improvement over the Hampshire U.S. Pat. No. 5,475,545 reference by using a variable velocity demand fraction value, thus allowing further tailoring of the resulting current that is applied to the VCM.
U.S. Pat. No. 4,965,501 issued to Hashimoto, discloses a different approach to reducing noise at initial stages of a seek. Instead of reducing the initial demand velocities as taught by the aforementioned references, the Hashimoto U.S. Pat. No. 4,965,501 initiates a seek by determining a large velocity error and generating a large initial current command (as with the standard velocity-controlled approach), but then sets an output servo gain block to a very low level and successively increments the gain to gradually apply more current to the VCM until the maximum velocity is reached.
It will be noted that a tradeoff must be made between performing the seek in as quickly a manner as possible and reducing the generation of acoustic noise during the seek. The foregoing references provide different methodologies to tailor the initial application of current to arrive at an acceptable compromise, and have been found operative in the art.
Nevertheless, as ever greater levels of operational performance are required to meet the demand for disc drives with ever increasing data storage capacities and transfer rates, there remains a continued need for improvements in the art whereby seeks can be carried out with reduced levels of acoustic noise and seek time variation. It is to such improvements that the present invention is directed.